They heal as much as they damage the target, and only for as much as they actually caused. So even when the combat log says that your Drain Life hit for 100, the player may only had 50 health left, hence you will be healed for 50. Absorb effects, such as Shadow Ward, Ice Barrier, and Power Word: Shield, will cause life drains to do no healing, although they do damage the target. This form of healing is now affected by healing reducing effects such as Mortal Strike. Their damage causes normal threat, whereas their healing does not generate any threat at all.

Life Drains are always binary effects, as such they can only hit or miss, there are no partial resists. Some creatures, generally constructs, are immune to life draining effects.

They generally have the same spell damage rules as normal spells. However, any spell damage is equally divided among the damage and the heal. So for pure damaging purposes, those spells get less interesting. Furthermore, these effects do only benefit from general spell damage and their respective school's spell damage. They are not affected by pure healing. Therefore, they always do as much healing as damage, with only a few exceptions (Vampiric Embrace's healing depends on the amount of players and talent points invested; Fel Armor increases the health gain from any spell).

Mana Drain spells are abilities that remove mana from the enemy. Normally this mana is used for a secondary effect. They cannot be used against Rogues, Warriors and Druids in Bear or Cat form.

There are different realizations. For example, the Warlock's Drain Mana transfers the mana to refill one's own mana pool. The Priest's Mana Burn and Feedback use the mana to cause damage to the enemy. All of these effects depend on the actual amount of mana drained from the enemy.

Neither the mana drain nor the caused damage do benefit from spell damage. These effects can only be raised by talents and debuffs which explicitly increase the drain or the respective school's damage.